Roseanne Warren

Highly active ruthenium oxide coating via ALD and electrochemical activation in supercapacitor applications

Highly active ruthenium oxide was uniformly coated on vertically aligned carbon nanotube forests for pseudocapacitor electrodes in enhanced energy storage applications. Atomic layer deposition (ALD) was designed to realize the conformal coating process onto porous structures and an electrochemical oxidation process was developed to achieve highly active ruthenium oxide. Results show 100× and 170× higher specific capacitance after the ALD coating and further electrochemical oxidation process, respectively, as compared with that of pure CNT electrodes. Furthermore, the measured capacitance value was close to the theoretical limit of ruthenium oxide at 644 F g−1 with a high power density at 17 kW kg−1. The electrode performance was tested over 10 000 charge–discharge cycles with gradually improved capacitance of 17% higher than the starting value and at ultra-high scan rates of up to 20 V s−1.

Rapid assembly of multilayer microfluidic structures via 3D-printed transfer molding and bonding

A critical feature of state-of-the-art microfluidic technologies is the ability to fabricate multilayer structures without relying on the expensive equipment and facilities required by soft lithography-defined processes. Here, three-dimensional (3D) printed polymer molds are used to construct multilayer poly(dimethylsiloxane) (PDMS) devices by employing unique molding, bonding, alignment, and rapid assembly processes. Specifically, a novel single-layer, two-sided molding method is developed to realize two channel levels, non-planar membranes/valves, vertical interconnects (vias) between channel levels, and integrated inlet/outlet ports for fast linkages to external fluidic systems. As a demonstration, a single-layer membrane microvalve is constructed and tested by applying various gate pressures under parametric variation of source pressure, illustrating a high degree of flow rate control. In addition, multilayer structures are fabricated through an intralayer bonding procedure that uses custom 3D-printed stamps to selectively apply uncured liquid PDMS adhesive only to bonding interfaces without clogging fluidic channels. Using integrated alignment marks to accurately position both stamps and individual layers, this technique is demonstrated by rapidly assembling a six-layer microfluidic device. By combining the versatility of 3D printing while retaining the favorable mechanical and biological properties of PDMS, this work can potentially open up a new class of manufacturing techniques for multilayer microfluidic systems.

Compact thermal model for phase change memory nanodevices

Ge2Sb2Te5 (GST)-based phase change memory (PCM) is a digital memory technology set to replace flash because of its greater scalability, lower programming power requirements, faster read-write times, and enhanced durability. Modeling efforts have focused on characterizing the coupled electrical, thermal, and phase-transition processes that define PCM switching events. Understanding the effects of device geometry and cell material properties on temperature distribution, heat flow, and thermal switching times is critical for design optimization. This work develops a compact thermal model that efficiently calculates transient and steady-state temperature scaling trends associated with GST thickness, width, and thermal conductivity. Compact model results indicate that there is an optimal phase-change layer thickness for maximizing cell peak temperature. Lowering GST thermal conductivity enhances PCM scalability by decreasing this optimal thickness and also shortens device programming times. In contrast, decreasing phase-change layer width increases cell peak temperature at the expense of programming speed. Thermal boundary resistance affects both spatial and temporal scaling trends.

Titanium Disulfide Coated Carbon Nanotube Hybrid Electrodes Enable High Energy Density Symmetric Pseudocapacitors

While electrochemical supercapacitors often show high power density and long operation lifetimes, they are plagued by limited energy density. Pseudocapacitive materials, in contrast, operate by fast surface redox reactions and are shown to enhance energy storage of supercapacitors. Furthermore, several reported systems exhibit high capacitance but restricted electrochemical voltage windows, usually no more than 1 V in aqueous electrolytes. Here, it is demonstrated that vertically aligned carbon nanotubes (VACNTs) with uniformly coated, pseudocapacitive titanium disulfide (TiS2 ) composite electrodes can extend the stable working range to over 3 V to achieve a high capacitance of 195 F g-1 in an Li-rich electrolyte. A symmetric cell demonstrates an energy density of 60.9 Wh kg-1 -the highest among symmetric pseudocapacitors using metal oxides, conducting polymers, 2D transition metal carbides (MXene), and other transition metal dichalcogenides. Nanostructures prepared by an atomic layer deposition/sulfurization process facilitate ion transportation and surface reactions to result in a high power density of 1250 W kg-1 with stable operation over 10 000 cycles. A flexible solid-state supercapacitor prepared by transferring the TiS2 -VACNT composite film onto Kapton tape is demonstrated to power a 2.2 V light emitting diode (LED) for 1 min.

ALD ruthenium oxide-carbon nanotube electrodes for supercapacitor applications

This work presents the first demonstration of atomic layer deposition (ALD) ruthenium oxide (RuO2) and its conformal coating onto vertically aligned carbon nanotube (CNT) forests as supercapacitor electrodes. Specific accomplishments include: (1) successful demonstration of ALD RuO2 deposition, (2) uniform coating of RuO2 on a vertically aligned CNT forest, and (3) an ultra-high specific capacitance of 100 mF/cm2 from prototype electrodes with a scan rate of 100 mV/s. Advantages of the ALD method include precise control of the RuO2 layer thickness and composition without the use of CNT-binder molecules. In addition to high capacitance, preliminary results indicate that the ALD RuO2-CNTs have good stability over repeated cycling. Besides its use in supercapacitors, ALD-RuO2 has potential NEMS applications: in biosensors and pH sensing [1], as a strong oxidative material in multiple chemical processes [2], and in catalytic reactions for photocatalytic systems [3].

Electrospinning of micro spiral fibers

We describe an easy way to form micro spiral structures resulting from buckling instabilities of an electro jet of a nanoscale polymer fiber of polyvinglpyrrolidone-Cu(NO3)2 (PVP-Cu(NO3)2) sol) and discuss the formation process. We control the morphologies of the fibers into spiral fibers, and free-standing hollow cylinders by connecting an opposite high voltage supply (−5 to −10 kV) on the collector. The microstructured surfaces were observed by scanning electron microscope (SEM). SEM analysis revealed the presence of spirals with diameters of approximately 20 to 30 μm. The structures formed by the nanofibers could be used in diverse fields of nanotechnology, such as micro planar inductor and nanochannels.

Stackable cow dung based microfabricated microbial fuel cells

μL-scale microbial fuel cell (μMFC) technology has the potential to serve as an efficient renewable energy harvester for a variety of applications ranging from, on chip devices to autonomous sensors in remote locations. However, low voltage and power outputs have restricted such microbial fuel cells (MFCs) from most practical applications. To bypass this limitation, we present a stackable microfabricated high-voltage cow dung-based μL-scale microbial fuel cell (CDFC) that utilizes a complex natural substrate (cow dung) and microstructures to attain higher voltages and power densities. Specifically utilizing micropillars which increased the electrode surface area 155 % compared to planar electrodes and a rich microbial consortium in the cow dung. Experimental results for the CDFC revealed open circuit potentials (OCPs) of 0.85±0.05 V, which represent the highest reported for a μMFC thus far. The CDFC was also found to produce power densities of 95±10 W/m3. By using two CDFCs stacked in series OCPs were increased by approximately 100%. These results suggest that the CDFC methodology represents a big step towards making μMFCs viable energy harvesters for both electronic and biological applications.

Graphene electrodes enhance performance for microliter scale microbial fuel cells

Recently, microliter-scale microbial fuel cells (μMFCs) have garnered significant interest as effective energy harvesters for low power biological and electronic systems. Although researchers have attained high current densities and columbic efficiencies from such fuel cells, low power outputs and working potentials caused by the use of Au/Cr electrodes have limited the implementation of μMFCs in practical applications. To overcome these limitations, here we present a graphene-based μMFC (G-MFC) that utilizes laser synthesized graphene electrodes to generate open circuit potentials (OCPs) of 0.8 ± 0.05 V and power densities of 1820 ± 10 W/m3. Furthermore, the G-MFC produces a maximum power output of 364 μW. The stack-able and low cost design of our G-MFC allows for a wide range of applications and also serves as a platform for repeatable electrode and substrate based testing. These results suggest that our G-MFC methodology could offer an effective route to achieve viable energy harvesters for low power systems.